1. Field of the Invention
The present invention relates to digital signal receiving apparatus which receive and demodulate digital modulated signals, and it particularly relates to a digital signal receiving apparatus which has the functions of receiving and demodulating signals modulated by the OFDM (Orthogonal Frequency Division Multiplexing) method used in the digital terrestrial broadcasting and so forth and of diversity combining for portable reception and mobile reception.
2. Description of the Related Art
OFDM method, as a modulation scheme superior in realizing high-quality transmission and improving the frequency utilization efficiency, has in recent years been proposed for systems that transmit image signals or sound signals. In OFDM method, a multiplicity of subcarriers are placed within the band of a single channel. It is highly resistant to interferences caused by ghosts or multipath and therefore enables excellent mobile reception. There are four kinds of modulation by the OFDM method, namely, DQPSK (Differential Quadrature Phase Shift Keying), QPSK, 16 QAM (Quadrature Amplitude Modulation) and 64 QAM, which have each a different mapping method. Also, these modulation methods, of which DQPSK is called a differential modulation method and the others synchronous modulation methods, differ from one another in the type and placement position of a pilot carrier to be inserted in the OFDM symbol.
On the transmission side of an OFDM method, a modulation processing that includes mapping, time interleaving and frequency interleaving by various modulation methods is performed on signals compressed into an MPEG method. On the demodulation side of the OFDM method, a demodulation processing is basically carried out in a sequence completely opposite to the processing on the transmission side. Namely, demodulation processings corresponding to the above-mentioned three kinds of processing are carried out in the order of frequency deinterleaving, time deinterleaving and demapping.
In mobile reception, transmitted radio waves come under the influence of fading, so that the received power fluctuates significantly, thereby making it difficult to maintain high-quality transmission. A technique called diversity reception is one for reducing quality degradation that may be caused by fading. In this technique, a plurality of independent signals are received and the fading fluctuation is reduced by using such signals properly to realize a high-quality transmission. This technique of reducing fading fluctuation is called the diversity combining method, which comes in three basic types, namely, selection combining, equal-gain combining and maximum-ratio combining. In the selection combining, in particular, the least degraded signal is selected and outputted from a plurality of received signals, and the other signals are not used. The selection combining is considered to best serve a practical purpose because the circuit structure of this selection combining is simpler than those of the other two combining methods.
Now, if a selection combining processing is carried out, one least degraded data is selected and outputted from a plurality of data. When this data has a deviation from the reference point on the constellation, a Viterbi decoding is performed in such a way as to include the magnitude of the deviation. Hence, a problem arises in which errors get accumulated in the metric computation when a soft-decision Viterbi decoding is performed.